1. Field of the Invention
The present invention relates to an audio signal processor and generator. More specifically, the present invention relates to an audio signal processor and synthesizer including a digital early reflection and reverberation simulator and corresponding operating method utilizing a reduced memory size through decimation and interpolation filters.
2. Description of the Related Art
Acoustical characteristics of musical venues, including the finest concert halls and auditoriums, are highly dependent on reverberation characteristics. Sounds produced in a concert hall are formed from original sound signals combined with echoes reflected and reverberated from multiple walls and surfaces of the hall. The reflected and reverberated signals produce the impression of space to a listener. The multiple combined signals vary in evoked response from annoyance or incomprehensibility for speech signals in a highly reverberant auditorium to ecstasy in the case of emotional romantic music in a well-designed concert hall. Music is most often played in a venue having a poor acoustic environment such as a home, an automobile, a multiple purpose auditorium for sporting events as well as performance events, and the like. The poor acoustic environment of these venues primarily relate to short reverberation times. One technique for improving sound quality in a space having a poor acoustic quality is to add a reverberation simulation special effect. Music recordings commonly include the addition of reverberation prior to distribution. Reverberation is added by a natural process such as recording in a concert hall or by adding sound from an artificial process such as a plate reverberator or a spring reverberator.
The first electronic reverberation simulators were designed using conventional analog circuitry. Analog reverberators are so difficult to design that designers commonly resort to reverberation using mechanical devices such as springs and special metal plates.
Development of digital circuitry greatly eases the problems in producing reverberation simulators. Digital reverberators are highly flexible and produce nearly any imaginable form of reverberation. A simple digital reverberator includes a delay element and a mixer for mixing delayed and undelayed sound signals, thereby generating a single echo. Multiple echoes are simulated in a digital reverberator by feeding a portion of the delayed output signal back to the input of the delay element, creating a sequence of echoes. Reverberation parameters for an echo include the duration of the delay and the relative amplitudes of the delayed and undelayed sounds.
A concert hall quality reverberation may be reproduced exactly by recording an impulse response of a selected concert hall and applying a transversal filter technique to a sound to be reverberated. Typical reverberations times of 2 seconds require usage of a filter that is 50 K to 100 K samples long, a size that is clearly impractical for implementation in an integrated circuit. However, many circuits created from delay elements, summers and multipliers produce a reverberation echo so long as the circuit is stable and does not oscillate.
A practical integrated circuit implementation of a concert hall quality reverberation simulator commonly includes several delay elements having unequal delay lengths. The values of the plurality of delay lengths, for example the placement of taps in a single delay line, determines the quality of sound of the simulator. A highly pleasing sound is produced by placing the taps according to an approximately exponential distribution but also a distribution in which the taps are placed at prime number locations. This structure of a reverberation delay line creates a maximum rate of echo amplitude growth.
High-quality audio processing and generation is heretofore achieved only in a system which includes a large amount of memory and which commonly includes more than one integrated circuit chip. Such a high-quality audio processing and reverberation system is cost-prohibitive in the fields of automotive acoustics, consumer electronics, consumer multimedia computer systems, game boxes, low-cost musical instruments and MIDI sound modules.
Implementation of reverberation simulation to greatly improve the quality of sound produced by a music synthesizer substantially increases the size of volatile or buffer storage. For example, a synthesizer which generates a 16-bit digital audio stream at 44. 1kHz typically employs a delay buffer size of about 32 Kbytes, an amount far higher than is feasible for implementation in low-cost and single-chip environments.
What is needed is a reverberation simulator having a substantially reduced memory size and computational load, and a reduced cost while attaining an excellent audio fidelity.